This disclosure relates to a monitoring system for a load bearing member, and more particularly to temperature compensation for monitoring a load bearing member, such as a roping arrangement for an elevator system.
Many elevator systems include an elevator car and counterweight suspended by a roping arrangement. The roping arrangement supports the load of the elevator car and facilitates movement of the car. A traction sheave causes movement of the roping arrangement to cause desired movement of the elevator car.
Elevator roping arrangements have included round steel ropes. Monitoring the condition of round steel ropes typically involves a manual, visual inspection. More recently, other types of load bearing members have been included in roping arrangements such as flat belts. With flat belts and other modern load bearing members, tension members may be at least partially encased in a jacket made of a material such as polyurethane. The jacket introduces a need for different inspection techniques.
There have been proposals to use electrical resistance-based inspection techniques for monitoring the condition of the tension members in an elevator load bearing assembly. With such techniques, changes in electrical resistance are interpreted as changes in the strength or load bearing characteristics of the tension member. One aspect of using resistance-based monitoring techniques is that electrical resistance may be influenced by conditions within the elevator hoistway. For example, temperature variations may cause changes in the apparent resistance of a tension member within the hoistway. One technique for addressing factors such as temperature is mentioned in U.S. Pat. No. 6,633,159. That document describes a technique in which the electrical resistance of one tension member within a flat belt is compared to the resistance of another. A possible shortcoming of that approach is that any tension members involved in the comparison are under the tension associated with the load of the elevator car and it may not always be possible to discern changes in resistance resulting from loading and use compared to a change caused by temperature.